Electric power cables of various designs for a number of different purposes and applications have been known for a long time. In particular, such cables usually comprise an armouring of a strong material that will contribute substantially to the mechanical strength and protection of the cable. Tensile strength is often essential in this connection, and steel of selected qualities is well known as an armouring material. Such armouring of known cable designs is wound as one or more outer layers in order to protect inner insulation layers and current conductors in the cables.
Whereas the types of cables referred to above are usually designed for being buried in the ground or to be laid on the sea bottom, mention should here also be made of overhead transmission lines that are suspended between masts for carrying very high voltage electric energy over long distances. Such transmission lines are known, where a kind of “armouring” is provided for in the form of a central steel core inside the conductor layers for current transmission, these conductor layers being mostly made of aluminium. Moreover, these high voltage transmission lines are not provided with any insulation layers along their length.
The present invention in one aspect relates to an electric power cable of a new design, in particular for submarine use in hydrocarbon pipeline heating systems and adapted to be installed on said pipelines, comprising a conductive core with an outer insulation and at least one protective sheath as well as armouring means.
The particular purpose of such cables is not comparable to the known cables referred to above, and even much more so when compared to the overhead transmission lines also mentioned above.
For heating of oil or gas pipelines in order to prevent hydrate and ice formation on the pipeline walls, the present applicant has developed a direct electrical heating system that is described, inter alia in British patent specification No. 2.373.321. For current supply to such a heating system a common practice is to install a current supply cable as a so called “piggyback” cable, which is traditionally installed simultaneously with the laying of the pipeline. More specifically such a cable is strapped to the pipeline during installation thereof. For a retrofit installation of a direct electric heating system, however, such simultaneous mounting of the current supply cable is not feasible.
In the case of cable that is piggybacked to the pipeline, the cable cannot have any metallic material such as armouring, outside the insulation layers, since this would make possible an undesired electrical path for return current, hence detracting from the effectiveness of the system. The return current referred to here, should of course as a whole flow through the pipeline walls in order to generate the heating effect aimed at.
The above considerations and in general the fact that installation and dynamic operation of deep sea power cables involve strict demands on the strength and bending properties of the cable, and specifically on the armouring material therein, constitute an important background for the present invention. Thus, there has been a need for a new electric power cable design that would meet the various criteria according to the above, in particular a solution with respect to cable strength and armouring, being also feasible in the case of a retrofit installation of piggybacked cables.